What is the optimum population size on earth?

These are the facts & projection:
In 2011 world's population has reached 7bn people. The current growth rate is exponential with adding 1bn people plus every 10 years. The peak is projected for 2050 to reach a level of 9.5bn people on earth.

My question is what is the optimum size of population on earth (and per continent)?

Just some examples, like we all know that bar of chocolate can make you happy but 50 or 100 every day will make you fat and depressive. Or running 5 miles every day will keep you fresh and sporty, but running two marathons every day will ruin your muscles & bones. So obviously there should be always an optimum or at least an optimum range like e.g. the habitable zone in our solar-system.

Oct 7 2013:
The main limiting factor is feeding everyone, so that would depend on food manufacturing technology.
There are estimates as per how many people the world can support today using current technology. If the population grows faster than the technology develops to support it (genetically modified crops mostly), we can expect the population to start declining at some point due to mass famines. It wouldn't be uniform across the planet however, seeing as food production and distribution is not homogenous.

I wouldn't hazard guessing a number, because I lack the expertise to make that calculation.
The bigger issue is how we control the population growth. These types of laws are, shall we say, less than popular.

Oct 7 2013:
And since technology depends on knowledge only and since knowledge has no limit, it follows that the maximum population size has no limit either.
One could argue that the optimal distribution should look like that to which we initially adapted... 134 000 000 km2 is the earth's surface available for habitat. That allows for about 200 000 tribes to live according to prehistoric standards, each tribe counting a reasonable 150 members.
So that's 30 million people in the entire world hunting and gathering food and resisting the temptation to plant anything or to find a cure for malaria.

Oct 7 2013:
Technology is by no means unlimited. You have this little thing called the laws of physics to worry about, and practical limitations on top of that.

As for the optimal distribution being the state of nature, let me remind you exactly what the state of nature entitles:
--a life expectancy of around 30 (combination of no medicine and constant warfare).
--living conditions so shoddy a cave starts looking decent.
--having to compete with wild animals over, well, everything.
--a small scale disaster as simple as a drought can kill everyone at the drop of a hat.
--eventual population growth beyond what the land can support naturally, forcing you to become a dirt farmer anyway or have everyone starve (a stone age dirt farmer lives in even worse conditions than a hunter gatherer--all the same problems but more work and a less diverse diet).

Oct 8 2013:
I prefer civilization too, I thought I made that obvious...
I have to disagree when you say the laws of physics constrain technological progress. What is the basis for such a confident argument?

Oct 8 2013:
Have you read/watched Rosling's talk in which he emphasized that regardless how much the knowledge and technology developed in the developed nations,there are still starvation and suffering in the undeveloped countries of the world. If you take the unsustainable population growth in countries like Bangladesh or Pakistan to some extent , we already reached the population size limit right now. So, the knowledge based limit doesn't quite make it a valid condition for the limit on population size unless some kind of free flowing exchange of the high technology AMONG ALL THE NATIONS can be established. The developed countries do try to buy dresses and other labor-intensive manufactured goods from Bangladesh, but that's just too little and inefficient for the alleviation of this hunger and inadequacy of other life necessities in those countries.

Oct 8 2013:
People starve for political reasons. Don't tell me we can't physically produce enough food to feed the planet with the technology we have right now. Politics too is something that can be improved with the right amount of knowledge and a way that allows progress to be made.

How do I go about explaining this? The laws of the universe say "this is how things work", and you build everything around that. If the laws of physics say "this can't be done", its not getting done, and that particular technology is never going to be developed.

Practical limitations are almost as bad. We've technologically speaking, had the capacity to replace everyone's cars with helicopters ever since the 1960's, but for a wide variety of practical reasons, it never happened (cost and difficulty in training people to fly aircraft mostly).
Our knowledge of flying technology has greatly increased since, and yet, we're all still using cars and not flying to work. Knowledge does not equate technology. Technology is practically applied scientific knowledge, and it has its limits.

Oct 8 2013:
The laws of the universe say how much we understand reality. So far we don't know how to shrink people, for instance. If we did, we could build the technology that allowed it and shrink everybody the size of an ant. Insufficient medical knowledge as well as computing knowledge makes that impossible today. But there is no law out there that can say it's impossible.
Knowledge does equal technology. If you know how to make flying cars easy to pilot and if you know how to build them for less than 15 dollars and if you know how to organize traffic so as to make it safe for everybody, then consider it done. We just lack the knowledge.
But perhaps we do for eternal reasons such as political ones or ethical ones, or cultural ones...
But hey I challenge you to give me an instance of technology that is impossible to build, independently of knowledge limitation.

I don't know what technology future humans will have access to, but so long as our current understanding of physics isn't fundamentally flawed, some things simply can't be done.

You've also got to be realistic about those things. A civilization who can't feed itself right this moment, for example, will probably collapse long before it can develop any new technology to save itself (three meals away from anarchy and all that).
Other things may remain completely impractical for hundreds of years to come, even things we know are possible. Humanity has known that flying was possible before the dawn of civilization (birds do it all the time after all), but it took a couple of thousands years to replicate that particular feat.

You can't just expect technology to come to your rescue, mostly because its practically impossible to predict more than a few years ahead.

Oct 8 2013:
Perpetual motion can be achieved if you can harness infinite energy. It's not easy and if you would kindly reduce 'perpetual' to 'billions of years', one could quickly solve the dilemma and come up with such a machine.
There are rules, but an infinite amount of solutions to work around them. Your example differs little from the rule that people cannot fly. With a little creativity and knowledge about flight you can get people in the air, although not in a way one had hoped for, at first.
I agree that one cannot rely confidently on unpredictible outcomes of scientific progress. There are other issues such as geopolitics to worry about. But belief in a limit of knowledge or technological improvement is not founded anymore than belief in divine intervention.

Oct 10 2013:
Very interesting question, and one that I think in the not-so-distant future the 'World Population and Reproductive Administration (WPRA)' will be very keen on monitoring. Factors will probably include: level of prosperity per person; resource distribution variance; habitational lifestyle balance; cultural/ideology balance; a general enjoyment index; etc. By then, people will be able to 'order' children per licensed limit. So there will no longer be uncontrolled population growth. Death for the unable and the infirm will probably be encouraged to help balance the 'healthy' population. Those of specifically identified 'favorable' characteristics will be supported in growth for the 'good of the society'. So the 'optimum size' will be a constantly adjusted algorithm similar to how we view economic output today. "...and we shall have peace." Emperor Palpatine

Oct 10 2013:
A good deal less than the amount of people we have at the moment. I think the planet is creaking quite a bit under the pressure already. I think that a population of say 3 billion is managable for the planet without causing major destruction as is happening now.

Oct 8 2013:
You are talking about the optimal population size of 30 million, but we are already having 7 billion going on to 9.5 billion.
Yes, starvation happened at least partially due to political reasons, but the same political forces also resist the measures of attempt to reduce population explosion as well . Let's look at Bangladesh and Pakistan, they have a combined population size of 340 million people which is slightly more than the whole population of the U. S. Furthermore, even if we are willing to give them enough food and a few other life necessities (which is not very likely), there are still practical difficulty to deliver smoothly to the people who are starving or hungry. For example, if we just give food to the government, probably less than half of the food would reach the hands of the hungry people. If we send our own workers or workers from the UN, it's very likely they will be harmed or killed in places like Pakistan by the militant Talibans. Or the Talibans will get most of the food anyway.
Furthermore, the phenomenon of slow or no growth of population will occur not only having the population barely getting fed, but you have to get them well educated and having a relatively comfortable life also.
We are talking about the population explosion here. You can see that it's not as simple as by just sharing of wealth by all the countries, which in turn is not necessarily a smooth undertaking either BECAUSE OF THE POLITICAL TURMOIL IN THE CURRENT WORLD SITUATION, OR BECAUSE OF HUMAN NATURE IN SELFISHNESS AND GREED.

Extract some human DNA, deep-freeze multiple samples and place them around earth in several stable orbits in space. Spread other samples across the universe, by accelerating them up to escape velocity, just in case the earth gets hit one day by something really big or when the sun dies out.

This way, all of us could happily die off because we fulfilled our purpose, and kept our DNA alive.

I think that there is a lot of truth to his comments.......it's the lack of expansion of the idea that gets me. Although I have tried to elicit more....I have failed miserably. Perhaps a language issue?
Don't know....... :-/

You know, sometimes I can't even spill a glass of water correctly, so thank you very much for your veto!

Are the brackets optional or part of the message? And what about harmful and invalid?

Is invalid (harmful) happiness the same or worse than (invalid happiness)?

At one of my first encounters I did follow the link to the whole paper he posted and tried my very best to understand at least the basic principles. I failed miserably as well.

Yet if I play nice, because I know that you like harmony, I have to admit that my own comments (tend) to follow their (own repetitive) pattern as we(ll). But I never use brackets to confuse people ... ;o)

Oct 7 2013:
Statistics state 1 billion (15%) of the world's population already live on the edge of starvation, with one person dying of hunger every 3.6 seconds. 33% are considered to live with hunger as a permanent fact of life. 11% of America's population are considered to live 'hungry' too. There's plenty of food produced in the world, there exists plenty of adequate technology for distribution, and about 70% of food in the West is wasted.
In other words, there's no "technical" reason why anyone in the world should be hungry or starving.

Perhaps it is because some groups profit from food scarcity?
Perhaps the human race is simply just incompetent, despite its advanced technologies?
Perhaps the starving can't access this TED conversation so they can tell us what kind of help they really need?
Perhaps there are greater historical forces at work (which we don't understand) and which determine the inevitability of the current state of affairs concerning world hunger?

Whatever the reason, no advance in technology will make any difference whatsoever until there are starving people banging on the doors of politicians in Washington, London, Tokyo, Paris, Brussels...etc.

If you don't believe me, read "The Great Stink of London" by Stephen Halliday. In 1858 the stench of sewage in the river Thames which flowed past the Houses of Parliament in London, got so bad parliament could no longer meet and conduct business. ONLY THEN was the long-standing problem of sewage disposal in London dealt with by politicians, immediately making funds available to build London's sewage system (still vitally in operation over 150 years later). The hero of the project by the way was Sir Joseph Bazalgette, one of the greatest of the Victorian engineers, and someone whom few people have heard of.

The optimum size of world population ... ? I suppose optimum implies "not starving". Well, given the political will, the world can be fed anytime, and in larger numbers.